President Obama warned Russia on Monday of possible U.S. sanctions over its military land grab in Ukraine, but Moscow brushed aside international threats, tightening its stranglehold on Crimea and calling audaciously for a national unity government in Kiev.

In Washington, Mr. Obama said the world is “largely united” against Russia’s military action and he is considering economic and diplomatic steps that would gradually isolate Russia. He criticized the government of Russian President Vladimir Putin for being “on the wrong side of history.”

“What cannot be done is for Russia with impunity to put its soldiers on the ground and violate basic principles that are recognized around the world,” Mr. Obama said. “Over time, this will be a costly proposition for Russia.”

But the U.S. and European Union floundered for solutions — while global markets panicked over the prospect of violent upheaval in the heart of Europe. Fears grew that the Kremlin might carry out more land grabs in pro-Russian eastern Ukraine, or elsewhere in the former Soviet Union, adding urgency to Western efforts to defuse the crisis.

Secretary of State John F. Kerry was heading to Kiev in an expression of support for Ukraine’s sovereignty, and the EU threatened a raft of punitive measures as it called an emergency summit on Ukraine for Thursday. In Congress, lawmakers prepared to move on an aid package for Ukraine.But it was Russia that appeared to be driving the agenda.

Foreign Minister Sergey Lavrov said in Geneva at the beginning of a U.N. Human Rights Council session that Ukraine should return to the Feb. 21 agreement signed by pro-Russian President Viktor Yanukovych — but not by Moscow — aimed at ending Ukraine’s governing crisis.

Mr. Yanukovych fled the country after sealing a pact with the domestic opposition — and with the foreign ministers of France, Germany and Poland — that called for early elections and the weakening of presidential powers.

“Instead of a promised national unity government,” Mr. Lavrov said, “a ‘government of the victors’ has been created.” He justified the Russian intervention in the Crimean peninsula as necessary for the protection of Russian citizens and ethnic Russians within Ukraine’s borders.

The U.S. ambassador to the U.N., Samantha Power, said Mr. Lavrov’s assertions were “without basis in reality.”

“What is happening today is a dangerous military intervention in Ukraine,” Ms. Power said. “It is an act of aggression. It must stop.”

The Pentagon said late Monday that it will suspend military exercises and other activities with Russia. Navy Rear Adm. John Kirby said the U.S. military has “put on hold” all military-to-military engagements, including bilateral meetings, port visits and planning conferences.

Meanwhile, Ukraine claimed that Russian troops had issued an ultimatum for two Ukrainian warships to surrender or be seized — prompting Ukraine’s acting president to accuse Russia of piracy.

Four Russian navy ships in Sevastopol’s harbor were blocking Ukraine’s corvette Ternopil and the command ship Slavutych, Ukrainian authorities said. Acting President Oleksandr Turchynov said commanders and crew were “ready to defend their ships. … They are defending Ukraine.”

Vladimir Anikin, a Russian Defense Ministry spokesman in Moscow, dismissed the report of a Russian ultimatum as nonsense but refused to elaborate.

It was not clear what the West could do to make Russia back down. The clearest weapon at the disposal of the EU and U.S. appeared to be economic sanctions that would freeze Russian assets and pull the plug on multibillion-dollar deals with the Kremlin. Late Monday, the EU threatened to freeze visa liberalization and economic cooperation talks and boycott the Group of Eight summit in Russia if Moscow does not back down on the Crimean peninsula by Thursday’s summit.

Ukrainian Prime Minister Arseniy Yatsenyuk appealed for outside help and said Crimea remained part of his country. “Any attempt of Russia to grab Crimea will have no success at all. Give us some time,” he said at a news conference with British Foreign Secretary William Hague.

European foreign ministers held an emergency meeting on a joint response to Russia’s military push.

Mr. Hague, like other Western diplomats, ruled out military action. “The U.K. is not discussing military options,” he said. “Our concentration is on diplomatic and economic pressure.”

After a 90-minute phone call between Mr. Obama and Mr. Putin over the weekend yielded no apparent results, the president said he understood the historic ties between Russia and Ukraine. But Mr. Obama said Ukrainians must determine their own future.

“The strong condemnation … from countries around the world indicates the degree to which Russia is on the wrong side of history,” Mr. Obama said.

The president said Russia has a choice of two paths. “Obviously, the facts on the ground in Crimea are deeply troubling, and Russia has a large army that borders Ukraine,” Mr. Obama said. “Now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force.”

The administration, along with the other members of the G-8 besides Russia, has halted preparations for the group’s annual June summit, this year scheduled for Sochi, Russia.

Mr. Obama said he is looking at a range of other steps that “will have a negative impact on Russia’s economy and status in the world.”

Lawmakers in both parties expressed interest in an aid package for Ukraine.

Sen. Robert P. Casey Jr., Pennsylvania Democrat, suggested loan guarantees and other economic assistance. House Majority Leader Eric Cantor, Virginia Republican, said he has directed House committee chairmen to develop a package of loan guarantees rapidly to show that “the United States will not stand idly by in the face of such aggression.”

Mr. Obama said such aid should be “the first order of business” when Congress resumes work Tuesday. “At this stage, there should be unanimity among Democrats and Republicans that when it comes to preserving the principle that no country has the right to send in troops to another country unprovoked,” the president said. “We should be able to come up with a unified position that stands outside of partisan politics.”

The threat of sanctions may not be enough to change Mr. Putin’s mind, analysts said.

Economic and diplomatic measures “are about all we can do right now,” said Steven Bucci, director of the Douglas and Sarah Allison Center for Foreign and National Security Policy at the Heritage Foundation.

“Is it good enough? I’m not terribly optimistic,” he said.

A powerful member of the Senate Armed Services Committee said Mr. Obama’s mismanagement of the defense budget is the reason that the U.S. has limited options for assisting Ukraine.

Sen. James M. Inhofe, Oklahoma Republican, said he had predicted that the U.S. would surrender its global influence “and leave a vacuum that will be filled by Russia.”

“I warned this day was coming, and it is here,” Mr. Inhofe said in a statement. “President Obama’s attempt to seek peace through apologetic diplomacy while defunding and dismantling our military has failed. Today, our enemies don’t fear us and our allies no longer respect us.”

By Monday, it was clear that Russia had effectively turned Crimea into a protectorate.

Russian soldiers controlled all Crimean border posts as well as all military facilities in the territory. Troops also controlled a ferry terminal in the Ukrainian city of Kerch, just 12 miles across the water from Russia. That intensified fears in Kiev that Moscow will use that route to send even more troops into the peninsula.

Border guard spokesman Sergei Astakhov said the Russians were demanding that Ukrainian soldiers and guards transfer their allegiance to Crimea’s new pro-Russian local government.

“The Russians are behaving very aggressively,” he said. “They came in by breaking down doors, knocking out windows, cutting off every communication.”

He said four Russian military ships, 13 helicopters and eight transport planes had arrived in Crimea in violation of agreements that permit Russia to keep its Black Sea fleet at the naval base in Sevastopol but limits the deployment of additional forces at the base.

Mr. Yatsenyuk, the Ukrainian prime minister, acknowledged that his country had “no military options on the table” to reverse Russia’s military move into Crimea because the Russians surrounded Ukrainian military facilities on the peninsula, completing a takeover without firing a shot.

• Maggie Ybarra contributed to this article, which is based in part on wire service reports.